Head mounted display with eye tracking

ABSTRACT

A head mounted display is disclosed. More particularly, a head mounted display including one or more projection light sources, one or more eye-tracking light sources, a polarizing beam splitter, and a second polarizing beam splitter is disclosed. Light from the one or more projection light sources and the one or more projection light sources and the one or more eye-tracking light sources are both at least partially reflected by the polarizing beam splitter. An optical path between the polarizing beam splitter and the second polarizing beam splitter passes through air. A head mounted display that utilizes polarizing beam splitters having certain reflection bandedges over a range of incidence angles is disclosed.

BACKGROUND

Head mounted displays, as a subset of wearable computing devices, oftenutilize projected light to display an image within the wearer's visualfield. Eye tracking systems may utilize a camera centered on thewearer's pupil or observe light reflected off a wearer's eye to detectgaze direction.

SUMMARY

In one aspect, the present disclosure relates to a head mounted opticaldevice. The head mounted optical device includes one or more projectionlight sources, one or more eye tracking light sources, a polarizing beamsplitter, and a second polarizing beam splitter. The optical device isconfigured such that projection light from the one or more projectionlight sources and eye-tracking light from the one or more eye-trackinglight sources are both at least partially reflected by the polarizingbeam splitter and such that an optical path between the polarizing beamsplitter and the second polarizing beam splitter for at least one ofprojection light from the one or more projection light sources andeye-tracking light from the one or more eye-tracking light sourcespasses through air.

In another aspect, the present disclosure related to a head mountedoptical device that includes one or more projection light sources, oneor more eye tracking light sources, a polarizing beam splitter, and asecond polarizing beam splitter. The optical device is configured suchthat projection light from the one or more projection light sources andeye-tracking light from the one or more eye-tracking light sources areboth at least partially reflected by the polarizing beam splitter. Asmeasured between 40 degrees and 50 degrees in a medium with a refractiveindex of about 1.53, the polarizing beam splitter has a right band edgebetween about 950 nm and about 850 nm. In some embodiments, as measuredbetween 40 degrees and 50 degrees in a medium with a refractive index ofabout 1.53, the second polarizing beam splitter has a right band edgebetween about 800 nm and about 700 nm.

At least one of the polarizing beam splitter and the second polarizingbeam splitter may include multilayer optical film. The eye-trackinglight from the one or more eye-tracking light sources may includeinfrared light. In some embodiments, the eye-tracking light from the oneor more eye-tracking light sources includes a substantial portion oflight with a wavelength between 700 nm and 1000 nm. In some embodiments,an eye-tracking wavelength range of the eye-tracking light from the oneor more eye-tracking light sources and a projection wavelength range ofthe projection light from the one or more projection light sources donot overlap. In some embodiments, the polarizing beam splitter reflectsat least 50% of a first polarization state but less than 50% of a secondorthogonal polarization state of eye-tracking light from the one or moreeye-tracking light sources. In some embodiments, the polarizing beamsplitter reflects at least 50% of both a first polarization state and asecond orthogonal polarization state of eye-tracking light from the oneor more eye-tracking light sources. In some embodiments, the polarizingbeam splitter reflects at least 50% of a first polarization state butless than 50% of a second orthogonal polarization state of projectionlight from the one or more projection light sources. In someembodiments, the second polarizing beam splitter transmits at least 50%of both a first polarization state and a second polarization state ofeye-tracking light from the one or more eye-tracking light sources. Thepolarizing beam splitter and the second polarizing beam splitter mayhave different right band edges. In some embodiments, the polarizingbeam splitter and the second polarizing beam splitter have substantiallyequal left band edges. In some embodiments, when measured at 45 degreesin a medium with a refractive index of about 1.53, the polarizing beamsplitter has a right band edge of about 900 nm. When measured at 45degrees in a medium with a refractive index of about 1.53, the secondpolarizing beam splitter may have a right band edge of about 750 nm. Insome embodiments, the head mounted optical device includes a quarterwave plate. In some embodiments, the head mounted optical deviceincludes an image sensor. The image sensor may include a CCD imager orit may include a CMOS imager. In some embodiments, the head mountedoptical device includes a frame, and the image sensor is disposed withinthe frame. The polarizing beam splitter may be immersed in a lens.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic top perspective view of a head mounted opticaldevice.

FIG. 2 is a schematic top perspective view of the head mounted opticaldevice of FIG. 1 including an eye tracking system.

FIG. 3 is a graph depicting the right reflection bandedge as a functionof incidence angle for several polarizing beam splitters.

FIG. 4 is a schematic top perspective view of another head mountedoptical device including an eye tracking system.

DETAILED DESCRIPTION

As a subset of wearable computing devices, head mounted displays maycorrespond in appearance to accessories such as glasses that are wornpublicly. Correspondingly, it may desirable when incorporating systemsinto a head mounted display to add minimal bulk and conspicuity.Similarly, interaction with and control of head mounted displays, againowing to its potential for public use, may be designed to be asinconspicuous and appear as natural in social settings as possible.Alternatively, head mounted displays may be desirable in industrialsettings when the wearable device has other purposes, such asoccupational safety. In this case, it may be advantageous to configurethe systems as to not expose sensitive electronics to the industrialenvironment. Eye tracking systems may be incorporated into head mounteddisplays in order to allow relatively unnoticeable eye movements to bedetected and interpreted. For example, a head mounted display may beconfigured to detect when a wearer's gaze is focused on a certainportion of the display.

In previous head mounted displays that included eye tracking, many use acamera positioned at the opposite side of the head mounted display fromthe projection optics, as described in United States Patent PublicationNo. 2013/0106674 A1 (Wheeler et al.) . This prevents both the eyetracking camera and the projection optics from being hidden within aframe and creates a noticeable object that may obscure part of awearer's field of view.

FIG. 1 is a top perspective view of a head mounted display. Head mounteddisplay 100 includes frame region 102 and viewing region 104. Frameregion 102 includes illuminator 110, polarizing beam splitter 120 withinmedium 122, and spatial light modulator 130. Viewing region 104 includessecond polarizing beam splitter 140 within second medium 142, quarterwave plate 150, and reflector 160. Eye 170 is shown to illustrate theoptical path of projection light from illuminator 110. For purposes ofthis application, the label “second” in “second polarizing beamsplitter” is used simply to distinguish the two polarizing beamsplitters by the order of their description; in other words, the labelis arbitrary and for ease of explanation only. In some embodiments, forexample, what is described herein as the second polarizing beam splittermay be equally be described as the polarizing beam splitter, and viceversa.

Illuminator 110 may be any suitable component or set of components forgenerating or emitting light. Illuminator may include one or more lightsources, including light emitting diodes (LEDs), cold cathodefluorescent lights (CCFLs), or incandescent light bulbs. Illuminator 110may be powered by any suitable mechanism, including by a battery.Illuminator 110 may include any combination of light sources, includinglight sources of different wavelength ranges. In some embodiments, thelight sources of illuminator 110 may generate white or substantiallywhite light. In some embodiments, illuminator 110 may generate polarizedor partially polarized light, or it may generate a certain distributionof polarization states. The configuration and construction ofilluminator 110 may depend on the desired performance characteristics,including luminance, battery life, and operating temperature. In someembodiments, the luminance and wavelength output of illuminator 110 maybe configurable, either directly by a wearer or automatically dependingon environmental conditions, such as time of day, ambient light, desiredbattery life performance, or temperature.

Light emitted by illuminator 110 is indicated in FIG. 1 by projectedlight 112. Projected light 112 is depicted as a ray for ease ofillustration, but it should be evident to those with skill in the artthat projected light 112 is representative of any suitable lightdistribution, including collimated or Lambertian distributions, lightcones, or the like. Suitable collimation optics may be included inconjunction with illuminator 110 to provide the desired lightdistribution of projected light 112.

Projected light 112 is incident on polarizing beam splitter 120 frommedium 122. Medium 122 may be any suitable optical medium. In someembodiments, medium 122 is substantially transparent to minimize opticalloss. In some embodiments, medium 122 is selected for durability orprotective characteristics in order to provide rigidity, warp, or impactresistance for the components of head mounted display 100. Medium 122may also be selected for its manufacturability, including its ability tobe injection molded. Medium 122 may have any index of refraction and maybe optically coupled to illuminator 110 to minimize losses throughFresnel reflection at the interface between the illuminator and themedium. Further, polarizing beam splitter 120 exhibits different opticalproperties, for example, different reflection bandedges at least in partas a function of the refractive index of the medium in which it isdisposed. Therefore, it may be desirable to carefully select medium 122based on the desired optical properties of the medium/polarizing beamsplitter system.

Polarizing beam splitter 120 may be formed from any suitable materialand may be any suitable shape or size. In some embodiments, polarizingbeam splitter 120 may be substantially planar, as depicted in FIG. 1. Insome embodiments, polarizing beam splitter 120 may be oriented such thatprojected light 112 is incident at 45°. Other orientations may bedesirable based on the optical geometry of head mounted display 100. Insome embodiments, the polarizing beam splitter substantially reflectslight of one polarization while substantially transmitting light havingthe orthogonal polarization state. Many different proportions orreflectivity and transmittance may be appropriate depending on theparticular application. In some embodiments, reflecting light of onepolarization may mean reflecting 50% or more of light having a certainpolarization state. In some configurations, reflecting light of onepolarization may mean reflecting 60%, 70%, 80%, 90%, 95% or even 99% oflight of one polarization. Similarly, transmitting light of anorthogonal polarization may mean transmitting more than 50% of light ofhaving an orthogonal polarization state. In some embodiments, 60%, 70%,80%, 90%, 95%, or even 99% of light having an orthogonal polarizationmay be transmitted. Polarizing beam splitter 120 may be described ashaving a pass axis and a block axis, with the pass axis and block axisbeing oriented substantially at a 90° angle from one another.

Polarizing beam splitter 120 may include a reflective polarizer. In someembodiments, polarizing beam splitter 120 includes a wire grid polarizeror a cholesteric reflective polarizer. The reflective polarizer may be abroadband reflective polarizer. In some embodiments, polarizing beamsplitter 120 may include a multilayer optical film reflective polarizer,including, for example, those described in U.S. Pat. No. 7,468,204(Hebrink et al.).

Depending on the configuration, orientation, and construction ofpolarizing beam splitter 120 and its surrounding medium, includingincident angle, polarizing beam splitter 120 may exhibit differenttransmission or reflection properties based on the wavelength orwavelengths of incident light. In some embodiments, there may be awavelength for a given angle of incidence where the polarizer ceases tobehave as a polarizing beam splitter; that is, for example, it maytransmit two orthogonal states of incident light. This wavelength for agiven incidence angle may be described as a bandedge. For a typicalpolarizing beam splitter there will be two bandedges: a left (or lowerwavelength) and a right (or higher wavelength) bandedge. Bandedges arealso described in conjunction with FIG. 3.

At least a portion of projected light 112 is transmitted throughpolarizing beam splitter 120. In some embodiments, a first portion ofprojected light 112 having a first polarization state is transmittedwhile another portion, in some cases having a second, orthogonalpolarization state, is reflected as rejected light 114. Rejected light114 may in some cases be directed toward a light absorbing material. Inembodiments of the present disclosure where projected light 112 is atleast partially polarized to align with the pass axis of the polarizingbeam splitter, a higher proportion of projected light 112 may passthrough polarizing beam splitter 120.

The portion of projected light 112 that is transmitted throughpolarizing beam splitter 120 is next incident on spatial light modulator130. Spatial light modulator may be any suitable component or device,and may have any suitable size. In some embodiments, spatial lightmodulator may be or include a digital micromirror device or a liquidcrystal on silicon configuration. Spatial light modulator 130 mayreflect projected light 112 as modulated light 116. In some cases,spatial light modulator 130 produces light having grayscale information.In other embodiments, spatial light modulator additionally produceslight having color information. Modulated light 116 may now havespatially dependent luminance and color values; in other words, spatiallight modulator may provide image information to modulated light 116. Insome embodiments, spatial light modulator 130 may be capable ofproducing an image in only one color at a time, instead relying on humanperception to blend different colored images relayed in rapidsuccession. Spatial light modulator 130 may be powered, driven, and/orconfigured by any suitable components, including one or moremicroprocessors, microchips, or other microdevices. In some embodiments,spatial light modulator 130 rotates the polarization of at least a partof projected light 112, so that in some cases at least a portion ofmodulated light 116 is reflected by polarizing beam splitter 120 insteadof transmitted.

Modulated light 116 travels from frame region 102 of head mounteddisplay 100 into viewing region 104 and second medium 142 of the headmounted display. Note that frame region 102 and viewing region 104 aredistinguished for ease of explanation, but do not necessarily need tohave any identifiable boundary or represent a substantive difference inmedium, shape, or size. The regions are labeled to orient head mounteddisplay 100 by describing a region which may be positioned to be closerto a frame. Similarly, for example, frame region 102 is distinguishedfrom viewing region 104 because frame region 102 may in some casescontain components positioned such that a viewer would not observe thatregion to be transparent.

In some cases, frame region 102 and viewing region 104 are separated byor include a gap; that is, there may be air or another low indexmaterial separating or within the two media. In other words, there maybe a gap between polarizing beam splitter 120 and second polarizing beamsplitter 140. In some embodiments the optical path between polarizingbeam splitter 120 and second polarizing beam splitter 140 passes throughair. Utilizing a gap may have certain physical advantages, such as loweroverall weight and better comfort and aesthetics. More designflexibility is also possible, because the system is not limited to therectilinear optics of a monolithic injection molded piece, such as theone described in U.S. Patent Publication No. 2013/0207887 A1. Such aconfiguration may also provide desirable optical properties, such asmore desirable levels of magnification for the projected (i.e., display)light. Further, in some embodiments including a gap, light may become atleast partially collimated; that is, light rays incident from the lowergap index of refraction to a higher index of refraction may be benttoward the normal, which may be desirable in some applications.

After entering viewing region 104 of head mounted display 100, modulatedlight 116 is incident on second polarizing beam splitter 140 from secondmedium 142. In some embodiments, second medium 142 may be the samematerial—even being a unitary construction—or it have the same index ofrefraction as medium 122. Second polarizing beam splitter 140 may beoriented such that modulated light 116 is incident at 45°. Secondpolarizing beam splitter 140 may be configured or tuned with some or allof the same considerations as described above for polarizing beamsplitter 120. In some embodiments, one or more bandedges of secondpolarizing beam splitter 140 may be the same as for polarizing beamsplitter 120. Second polarizing beam splitter 140 may be configured totransmit most or all of modulated light 116, that is, to transmit thelight reflected by polarizing beam splitter 120. This may mean that thepass axis of polarizing beam splitter 120 and the pass axis of secondpolarizing beam splitter 140 are oriented substantially orthogonally toone another. In other words, polarizing beam splitter 120 and secondpolarizing beam splitter 140 may be crossed.

Modulated light 116, after at least partially transmitted through secondpolarizing beam splitter 140 in incident on quarter wave plate 150disposed on reflector 160. Modulated light 116 passes through quarterwave plate 150, is reflected by reflector 160, and passes back throughquarter wave plate 150. Quarter wave plate 150 may be any suitablequarter wave retarder, including suitable birefringent materials orliquid crystal layers. The thickness of quarter wave plate 150 may beselected to provide acceptable polarization rotation performance for awavelength or a set of wavelengths. Quarter wave plates are generallyconfigured to change the polarization state from linear to circularpolarization, or vice versa. Passing through a quarter wave plate twicemay have the same effect on the polarization state of light as passingthrough a half wave plate; that is, the polarization state may berotated 90°. Reflector 160 may be any suitable reflector, including astandard mirror or a multilayer optical film reflector, such as EnhancedSpecular Reflector (ESR) available from 3M Company, St. Paul, Minn.

Modulated light 116, after passing through quarter wave plate 150 twice,may have its polarization state rotated, becoming rotated light 118. Insome embodiments, this may cause rotated light 118 to be substantiallyor at least partially reflected by second polarizing beam splitter 140instead of transmitted (as modulated light 116, having an orthogonalpolarization state, was). Rotated light 118, after being reflected bysecond polarizing beam splitter 140, passes out of second medium 142 andinto the surrounding air, and is eventually observed by a viewer througheye 170. In some embodiments, the geometry and design of the interfacebetween second medium 142 and air may be designed or configured tocompensate for refraction as the light travels from an area of higherrefractive index to an area of lower refractive index.

FIG. 2 schematically illustrates a top perspective view of the headmounted optical device of FIG. 1 including an eye tracking system. Headmounted display 200 is similar to head mounted display 100 as depictedin FIG. 1. Corresponding with FIG. 1, head mounted display 200 includes,separated into frame region 202 and viewing region 204, illuminator 210,polarizing beam splitter 220 in medium 222, spatial light modulator 230,second polarizing beam splitter 240 in medium 242, quarter wave plate250, and reflector 260. The description and operation of thesecomponents for projecting light to eye 270 is not shown in FIG. 2 forease and clarity of illustration. Head mounted display 200 also includesimage sensor 280 including lens 282, and eye-tracking light sources 290.

Eye-tracking light sources 290 may be any number of light emittingcomponents. In some embodiments, eye-tracking light sources 290 mayinclude one or more LEDs. In some embodiments, eye-tracking lightsources 290 may emit light at least partially outside the visiblespectrum. Eye-tracking light sources 290 may emit infrared light. Insome embodiments, eye-tracking light sources 290 may emit light havingmultiple discrete wavelengths or a range of wavelengths. In some cases,eye-tracking light sources 290 may emit at least partially polarizedlight. Eye-tracking light sources 290 may be configured on head mounteddisplay 200 such that the eye-tracking light sources are inconspicuousand non-obscuring.

Eye-tracking light sources 290 emit light represented by eye-trackinglight 292. Eye-tracking light 292 is later incident on at least aportion of eye 270. Eye-tracking light sources 290 may be configured orpositioned such that the incidence angle of eye-tracking light 292 oneye 270 is low; in some cases as low as practically possible given otherdesign considerations.

Eye-tracking light 292 may be incident on one or more portions of eye270. The reflections at several refractive index interfaces at andwithin the eye are generally referred to as Purkinje images, of whichthere are four. For example, light reflected by the cornea of the eye isreferred to as the first Purkinje image, while light reflected by theback of the lens (of the eye) is referred to as the fourth Purkinjeimage. In some embodiments, eye-tracking light 292 may be configured toreflect at one or more of these interfaces, resulting in Purkinje light294. Purkinje light 294 may, as described elsewhere, include one or moreof the Purkinje images generated by the reflection of light atinterfaces of the eye.

Purkinje light 294 enters second medium 242 and is incident on secondpolarizing beam splitter 240. As described for head mounted display 100in FIG. 1, the outer geometry of head mounted display 200 may beconfigured to compensate for refraction caused by the change in mediumat the interface between air and second medium 242. Second polarizingbeam splitter 240 may be configured to at least partially reflectPurkinje light 294 having a certain polarization state. In someembodiments, second polarizing beam splitter 240 may be configured toreflect most or all of Purkinje light 294. In embodiments where at leastsome of Purkinje light 294 is transmitted through second polarizing beamsplitter 240, the light continues through the other side of viewingregion 204. In some embodiments, it may be important to minimize theselosses by controlling the initial polarization of eye-tracking light 292(and thereby Purkinje light 294).

Purkinje light being reflected by second polarizing beam splitter 240 isincident on quarter wave plate 250 and reflector 260. As for projectedlight (modulated light 116) described in FIG. 1, passing through quarterwave plate 250 twice may rotate the polarization of Purkinje light 294by 90°, represented by rotated Purkinje light 296. Rotated Purkinjelight 296 is incident on second polarizing beam splitter 240 and is atleast partially transmitted. In some embodiments, most or all of rotatedPurkinje light 296 is transmitted by second polarizing beam splitter.

Rotated Purkinje light 296 then enters medium 222 and is incident onpolarizing beam splitter 220. In some embodiments, rotated Purkinjelight 296 is also transmitted by polarizing beam splitter 220. Note thatin some embodiments described in FIG. 1, the pass axis for polarizingbeam splitter 120 (corresponding with polarizing beam splitter 220) andthe pass axis of second polarizing beam splitter 140 (corresponding withsecond polarizing beam splitter 240) are orthogonal. Thus, in order forboth polarizing beam splitters to pass rotated Purkinje light 296 of thesame polarization state, polarizing beam splitter 220 may in someembodiments at least partially transmit light of both polarizationstates. In some embodiments, polarizing beam splitter 220 maysubstantially pass light of a first polarization state and substantiallyreflect light of a substantially orthogonal polarization state for lighthaving a first wavelength or wavelength range, and transmit light ofboth polarization states for light having a second wavelength orwavelength range, where the first wavelength or wavelength range isdifferent from the second wavelength or wavelength range. In someembodiments, polarizing beam splitter 220 and second polarizing beamsplitter 240 may have different right bandedges. This may, in somecases, correspond to orthogonal polarization states of infrared lightbeing substantially transmitted by a polarizing beam splitter while onlyone polarization state of visible light is substantially transmitted.The other polarizing beam splitter may substantially transmit only onepolarization state of both infrared and visible light. These bandedgeprofiles are discussed in conjunction with FIG. 3, below.

Rotated Purkinje light 296 passes through lens 282 and may be focused,concentrated, or inverted, creating Purkinje image 298 on image sensor280. In some embodiments, lens 282 may include a filter, including afilter that passes only infrared light. As described elsewhere, Purkinjeimage 298 may include one or more of the four Purkinje images reflectedfrom eye 270. Image sensor 280 may be or include a passive-pixel sensor,such as a charge-coupled device, or CCD. In some embodiments, imagesensor 280 may include an active-pixel sensor, such as a sensorincluding a complementary metal-oxide-semiconductor (CMOS). Any suitableelectronics may accompany image sensor 280, including one or moreprocessors (not shown) to interpret the location and character ofPurkinje image 298. The one or more processors may transmit or modifyinformation based on Purkinje image 298. For example, the one or moreprocessors may make a portion of the projected light brighter based onwhere the processor detects the viewers gaze is focused. In someembodiments, the one or more processors may also or alternativelydetermine pupil size, change in pupil size, eye movement, or eyemoisture. This information may be used to determine a viewer's interestor reaction to certain content. In some embodiments the processor mayreceive input commands to interact with head mounted display 200 basedon gestures that utilize gaze direction.

FIG. 3 is a graph depicting the relationship between (right) reflectionbandedge and incidence angle for two polarizing beam splitters in a 1.53index medium. For some applications, given the geometry of the opticalsystem, the position of the bandedge at 45° may be of particularinterest. First polarizing beam splitter, depicted by line 310, has aright bandedge at 45° of about 900 nm. This polarizing beam splitter mayreflect one of each orthogonal polarization state for both visible lightand at least a portion of the near infrared spectrum, for example, 825nm light. Second polarizing beam splitter, represented by line 320, hasa right bandedge at 45° incidence of about 750 nm. This polarizing beamsplitter may reflect one of each orthogonal polarization state forvisible light but may substantially transmit both states of at least aportion of the near infrared spectrum, for example, 825 nm light.

In some embodiments, angles of incidence at 45° may represent only anideal case and the reflection bandedges over a more realistic or widerrange of angles may result in lost light and color defects. For example,light generated by an LED having a generally Lambertian distribution maybe collimated by various optical elements as to approach perfectcollimation, yet will never reach it. Further, in many cases, efforts toperfectly collimate Lambertian light often results in unacceptablebrightness and efficiency losses, which may, for example, contribute toa diminished battery life. In other words, actual angles of incidence onone or more reflective polarizers, even for a substantially rectilinearhead mounted display, may more realistically be characterized and beimportant over a range of between 40 and 50 degrees, or between 30 and60 degrees. Therefore, in some embodiments, because reflection bandedgesshift with incidence angle, failing to consider and design for thereflection bandedge's variation over a range of angles may result inunacceptable optical performance in many applications; for example, apolarizing beam splitter that is designed to transmit both states ofinfrared light at a 45° incidence may begin to reflect one or morepolarization states of infrared light over a more realistic range ofangles. Certain reflective polarizers, such as those with theperformance shown in FIG. 3 or, for example, Advanced Polarizing Film(APF) available from 3M Company, St. Paul, Minn., may be more suitablefor a realistic range of incidence angles.

The labels for first and second polarizing beam splitters described inconjunction with this graph are for the convenience of distinguishingthe two and do not necessarily correspond to the polarizing beamsplitter and the second polarizing beam splitter described in FIGS. 1-2.The values and curve profiles shown are exemplary and may be altereddepending on the desired application or specific configuration of thepolarizing beam splitter.

FIG. 4 is schematic top perspective view of another head mounted opticaldevice including an eye tracking system. Head mounted display 400includes frame 402, projection optics 410, immersed polarizing beamsplitter 420 in lens 422, eye-tracking light sources 440, and imagesensor 450 including lens 452.

Frame 402 may be any suitable shape and size or made from any suitablematerial. In some embodiments, frame 402 may be configured to appear asa standard glasses frame. In some embodiments, frame 402 may beconfigured to conceal at least some of the components of head mounteddisplay 400. Projection optics 410 are not shown in detail in FIG. 4 butmay be any suitable configuration or set of components, including, forexample, a digital micromirror projection system or a liquid crystal onsilicon system. Depending on the desired application of head mounteddisplay 400 projection optics 410 may produce projection light of asingle wavelength or projection light having multiple discretewavelengths or a range of wavelengths. In some embodiments, projectionoptics 410 may produce at least partially polarized light. Projectionoptics 410 may also contain a polarizing beam splitter. Note that, forthe head mounted display shown in FIG. 4, in these cases the opticalpath between the polarizing beam splitter included in projection optics410 and immersed polarizing beam splitter 420 passes through air.

Immersed polarizing beam splitter 420 may behave as any of thepolarizing beam splitters described elsewhere in the present disclosure;it may substantially reflect one polarization of light whilesubstantially transmitting an orthogonal polarization or it may insteador additionally substantially transmit both polarizations of light in acertain wavelength range. In some embodiments it may also substantiallyreflect both polarizations of light in a certain wavelength range.Immersed polarizing beam splitter 420 may be configured such thatemitted light from projection optics 410 has the right incidence anglein order to provide a desired optical performance. Immersed polarizingbeam splitter 420 along with lens 422 may be curved or shaped such thatlight incident from projection optics 410 is reflected as to be viewableat eye 430.

Eye-tracking light sources 440 may be any appropriate number and anysuitable type of light source, including those described foreye-tracking light sources 290 in conjunction with FIG. 2. Eye-trackinglight sources 440 may emit light in the infrared spectrum. In someembodiments, light from eye-tracking light sources 440 may be configuredwith immersed polarizing beam splitter 420 and lens 422 such that thegeometry causes light emitted from eye-tracking light sources 440 to beat least partially reflected by immersed polarizing beam splitter 420and directed into eye 430, where the reflected Purkinje image isdirected back through lens 452, where the image may be focused orconcentrated onto image sensor 450. As with image sensor 280 in FIG. 2whose description applies to this embodiment, image sensor 450 mayinclude suitable components and electronics, including CMOS and CCDsensors.

Embodiments of the present disclosure may be suitable for incorporationinto many head mounted optical displays, devices, or other near-to-eyeor wearable computers. Note that in some embodiments, because apolarizing beam splitter located in front of the eye at least partiallytransmits visible light of one or both orthogonal polarization states, aviewer may observe real world scenes through the head mounted display.In some embodiments where a polarizing beam splitter located in front ofthe eye at least partially reflects a polarization state of light, thehead mounted display may have the added benefit of reducing glare fromreflected light.

The following are a list of items of the present disclosure:

Item 1 is a head-mounted optical device, comprising:

-   -   one or more projection light sources;    -   one or more eye-tracking light sources;    -   a polarizing beam splitter; and    -   a second polarizing beam splitter;    -   wherein the optical device is configured such that projection        light from the one or more projection light sources and        eye-tracking light from the one or more eye-tracking light        sources are both at least partially reflected by the polarizing        beam splitter; and    -   wherein the optical device is configured such that an optical        path between the polarizing beam splitter and the second        polarizing beam splitter for at least one of projection light        from the one or more projection light sources and eye-tracking        light from the one or more eye-tracking light sources passes        through air.

Item 2 is a head-mounted optical device, comprising:

-   -   one or more projection light sources;    -   one or more eye-tracking light sources;    -   a polarizing beam splitter; and    -   a second polarizing beam splitter;    -   wherein the optical device is configured such that projection        light from the one or more projection light sources and        eye-tracking light from the one or more eye-tracking light        sources are both at least partially reflected by the polarizing        beam splitter; and    -   wherein as measured between 40 degrees and 50 degrees in a        medium with a refractive index of about 1.53, the polarizing        beam splitter has a right band edge between about 950 nm and        about 850 nm.

Item 3 is the head-mounted optical device of item 2, wherein as measuredbetween 40 degrees and 50 degrees in a medium with a refractive index ofabout 1.53, the second polarizing beam splitter has a right band edgebetween about 800 nm and about 700 nm.

Item 4 is the head-mounted optical device as in either item 1 or item 2,wherein at least one of the polarizing beam splitter and the secondpolarizing beam splitter include multilayer optical film.

Item 5 is the head-mounted optical device as in either item 1 or item 2,wherein the eye-tracking light from the one or more eye-tracking lightsources includes infrared light.

Item 6 is the head-mounted optical device as in either item 1 or item 2,wherein the eye-tracking light from the one or more eye-tracking lightsources includes a substantial portion of light with a wavelengthbetween 700 and 1000 nm.

Item 7 is the head-mounted optical device as in either item 1 or item 2,wherein a eye-tracking wavelength range of the eye-tracking light fromthe one or more eye-tracking light sources and a projection wavelengthrange of the projection light from the one or more projection lightsources do not overlap.

Item 8 is the head-mounted optical device as in either item 1 or item 2,wherein the polarizing beam splitter reflects at least 50% of a firstpolarization state but less than 50% of a second orthogonal polarizationstate of eye-tracking light from the one or more eye-tracking lightsources.

Item 9 is the head-mounted optical device as in either item 1 or item 2,wherein the polarizing beam splitter reflects at least 50% of both afirst polarization state and a second orthogonal polarization state ofeye-tracking light from the one or more eye-tracking light sources.

Item 10 is the head-mounted optical device of item 9, wherein thepolarizing beam splitter reflects at least 50% of a first polarizationstate but less than 50% of a second orthogonal polarization state ofprojection light from the one or more projection light sources.

Item 11 is the head-mounted optical device as in either item 1 or item2, wherein the second polarizing beam splitter transmits at least 50% ofboth a first polarization state and a second orthogonal polarizationstate of eye-tracking light from the one or more eye-tracking lightsources.

Item 12 is the head-mounted optical device as in either item 1 or item2, wherein the polarizing beam splitter and the second polarizing beamsplitter have different right band edges.

Item 13 is the head-mounted optical device as in item 12, wherein thepolarizing beam splitter and the second polarizing beam splitter havesubstantially equal left band edges.

Item 14 is the head-mounted optical device of item 1, further comprisinga quarter wave plate.

Item 15 is the head-mounted optical device of item 1, further comprisingan image sensor.

Item 16 is the head-mounted optical device of item 15, wherein the imagesensor includes a CCD imager.

Item 17 is the head-mounted optical device of item 15, wherein the imagesensor includes a CMOS imager.

Item 18 is the head-mounted optical device as in any one of items 15-17,further comprising a frame, wherein the image sensor is disposed withinthe frame.

Item 19 is the head-mounted optical device of item 1, wherein thepolarizing beam splitter is immersed in a lens.

All U.S. patents and publications cited in the present application areincorporated herein by reference as if fully set forth. The presentinvention should not be considered limited to the particular embodimentsdescribed above, as such embodiments are described in detail in order tofacilitate explanation of various aspects of the invention. Rather, thepresent invention should be understood to cover all aspects of theinvention, including various modifications, equivalent processes, andalternative devices falling within the scope of the invention as definedby the appended claims and their equivalents.

What is claimed is:
 1. An optical device, comprising: a first polarizingbeam splitter; and a second polarizing beam splitter facing, and adaptedto receive light transmitted and reflected by, the first polarizing beamsplitter, the first and second polarizing beam splitters havingsubstantially equal left band edges and different right band edges. 2.The optical device of claim 1 further comprising a wave plate forrotating a polarization state of light, the second polarizing beamsplitter disposed between the first polarizing beam splitter and thewave plate.
 3. The optical device of claim 1 further comprising areflector for reflecting light, the second polarizing beam splitterdisposed between the first polarizing beam splitter and the reflector.4. The optical device of claim 1, wherein for light incident at about 45degrees, the first polarizing beam splitter has a right band edgebetween about 950 nm and about 850 nm.
 5. The optical device of claim 1,wherein for light incident at about 45 degrees, the second polarizingbeam splitter has a right band edge between about 800 nm and about 700nm.
 6. The optical device of claim 1 further comprising a first lightsource configured to emit visible light and a different second lightsource configured to emit infrared light.
 7. The optical device of claim1 further comprising a light source configured to emit light away fromboth the first and second polarizing beam splitters, wherein lightemitted by the light source is received by the second polarizing beamsplitter after it is reflected from a viewer's eye.
 8. The opticaldevice of claim 1 further comprising first and second light sources,wherein wavelength ranges of light emitted by the first and second lightsources do not overlap.
 9. The optical device of claim 1, wherein thefirst polarizing beam splitter reflects at least 50% of a visible lighthaving a first polarization state and at least 50% of an infrared lighthaving an orthogonal second polarization state.
 10. The optical deviceof claim 1, wherein at least one of the first and second polarizing beamsplitters is immersed in a lens.
 11. The optical device of claim 1,wherein at least one of the first and second polarizing beam splittersincludes a multilayer optical film.